This invention relates to a stent used in conjunction with a magnetic resonance (MR) system, and more particularly to a stent which is capable of repeatedly ablating hyperplastic tissue growing around the stent when the stent is subjected to an RF electromagnetic field produced by an external scanner, thus preventing blockage of the stent.
Magnetic Resonance (MR) surface resonators are currently used in a variety of clinical and research applications. The purpose of a surface resonator is to provide improved signal-to-noise performance when imaging small regions. Typically, the resonator is placed on the surface of the body over the region of interest. The surface resonator can be used as a transmit/receive antenna, or, as in many applications, the volume resonator (sometimes referred to as a body coil) of the MR scanner will be used as the transmit antenna, while the surface resonator acts as the receive antenna to collect the MR signal from the desired region alone.
One area of concern when designing MR surface resonators for use as receive antennas is the decoupling issue. More specifically, if a surface resonator with the same resonant frequency as the transmit field is placed inside a volume resonator while the volume resonator is transmitting, the surface resonator will receive and retransmit an intense field around itself. This retransmitted field can result in RF burns to the patient. In order to prevent surface burns, the surface resonator is xe2x80x9cdecoupledxe2x80x9d during the volume resonator transmit procedure. The surface resonator is decoupled by causing its resonant frequency to change during volume resonator transmit. A diode switching circuit is used to add an additional reactive element to the resonant circuit while transmit is taking place. For example, an additional inductor added to a resonant circuit will lower the resonant frequency. When the resonant frequency of the surface resonator is sufficiently far from the transmit frequency, the surface resonator will not receive and retransmit a signal, and the RF burn hazard is eliminated.
Known treatments for removing or preventing hyperplastic tissue located within an implanted stent body utilize invasive procedures, such as inserting a catheter into the area near the stent. The catheter is designed to include an antenna at its terminal end. The catheter can then be used as a radio frequency transmit path for ablating tissue that could otherwise create blockage within the vessel. However, such invasive procedures are significantly complex, present higher risks of post procedure complications and can be very uncomfortable for the patient. There are also limits on the number of catheter procedures which can be performed on a patient who is more susceptible to a hyperplastic response.
In one aspect, the invention features a stent device. The stent device includes an electrically conductive helical structure. The stent device also includes an electrically conductive ring structure connected to the helical structure. The ring structure includes an inner conducting ring, an outer conducting ring, and a dielectric material disposed between the inner and outer conducting rings. The helical structure and the ring structure are arranged to produce an electromagnetic field when subjected to an applied electromagnetic field.
Embodiments may include one or more of the following features. The ring structure is connected to a first end of the helical structure, and further includes a second ring structure connected to a second, opposite end of the helical structure. The inner conducting ring, the outer conducting ring, and the dielectric material disposed between the inner and outer conducting rings are arranged for defining an electrical capacitor. The inner conducting rings of the ring structures are connected to the first end and the second end of the helical structure, respectively, and further include a return path conductor electrically interconnecting the first ring structure and the second ring structure. The return path conductor is connected to the outer conducting ring of each ring structure.
The helical structure defines a solenoidal inductor for conducting an electrical current. The helical structure and the ring structure define an electrically reactive circuit having a resonant frequency. The helical structure and the ring structure produce the electromagnetic field at the resonant frequency. In one configuration, the helical structure and the inner and outer conducting ring of each ring structure are formed from a nickel-titanium alloy. The nickel-titanium alloy comprises about 40% to 60% nickel.
In another aspect, the invention features a stent for implantation into a vessel of a body. The stent includes a solenoidal inductor formed by a helical wire structure and a capacitor is connected at each end of the inductor. A return path conductor electrically interconnects the capacitors. The inductor and the capacitor are arranged to radiate an electromagnetic field when subjected to an applied electromagnetic field.
Embodiments may include one or more of the following features. Each capacitor includes an inner conducting ring, an outer conducting ring, and a dielectric material disposed between the inner and outer conducting rings. The inner conducting rings of the capacitors are connected to a first end and a second end of the helical wire structure, respectively, and the return path conductor is connected to the outer conducting rings of the capacitors. The conductor is electrically connected in parallel with the capacitors. The inductor, the inner conducting ring and outer conducting ring of each capacitor, and the return path conductor are formed from a nickel-titanium wire structure. The inductor and the capacitors define an electrically reactive circuit having a resonant frequency. The applied electromagnetic field is transmitted by a magnetic resonance transmitter at the resonant frequency. The electrically reactive circuit radiates the electromagnetic field at the resonant frequency.
In another aspect, the invention features an RF reactive stent for implantation into a vessel of a body. The stent includes a solenoidal inductor for conducting an induced current. The inductor is formed from a helical wire structure having a first end and a second end. A first capacitor is connected to the first end of the inductor, and a second capacitor is connected to the second end of the inductor. A return path conductor electrically interconnects the first capacitor, the second capacitor and the solenoidal inductor as an electrically reactive circuit. The electrically reactive circuit forming the stent has a resonant frequency. The first capacitor, the second capacitor and the solenoidal inductor are arranged to generate an RF field when subjected to an applied RF field at the resonant frequency of the stent.
Embodiments may include one or more of the following features. The first and second capacitors are each formed by a ring structure having an inner conducting ring, an outer conducting ring, and a dielectric material disposed between the inner and outer conducting rings. The inductor, the first and second capacitors, and the return path conductor are coated with an insulating material. In one configuration, the insulating material is a polymer. The inductor, the inner and outer conducting rings of each capacitor, and the return path conductor are formed from a nickel-titanium wire structure.
In another aspect, the invention features a method for ablating tissue surrounding a reactive stent device. The method includes the steps of providing an RF reactive stent formed from a solenoidal inductor element which is electrically interconnected to a capacitor element. The method also includes implanting the stent within a vessel of a body, and irradiating the stent with an applied RF field for causing the inductor element and the capacitor element to generate an RF field in the vessel.
Embodiments may include one or more of the following features. The stent has a resonant frequency and the stent is irradiated by the applied RF field at the resonant frequency. The method includes the step of identifying the resonant frequency associated with the stent after the step of implanting the stent. The RF field generated by the inductor element and the capacitor element causes heating of tissue forming the vessel. The step of irradiating the stent produces a selected amount of heat sufficient to cause ablation of the tissue, which may be hyperplastic tissue surrounding the stent.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.